Fire detecting system

ABSTRACT

A fire detecting system having a voltage source, a pair of conductors extending from the source with a plurality of fire detectors connected across the conductors, means at the end of said conductors placing a constant load on the voltage source and means adjoining the voltage source for detecting loss of said load resulting from a break in one of said conductors.

United States Patent Yuasa Mar. 12, 1974 FIRE DETECTING SYSTEM 3,056,1209/1962 Mobarry 340/409 ux 4 1751 Inventor: Hayami Tokyo, Japan 312E132;12/133 2121???? 343/183 73 Assignee; in Company, Limited, Tokyo,3,643,247 2/1972 Ratcliffe 340/256 X Ja an p FOREIGN PATENTS ORAPPLICATIONS [22] Flled: 1972 1,002,291 8/1965 Great Britain 340/253[21] Appl. No.: 219,723

Primary Examiner-John W. Caldwell [30] Foreign Application Priority DataAssistant Examiner-William M. Wannisky Feb. 4, 1971 Japan 46-4032 52 us.c1 340/227, 340/253 R, 340/409, [57] ABSTRACT [51] I t Cl 307/285 f iZQZA fire detecting system having a voltage source, a pair [58] i i C 248 Bof conductors extending from the source with a pluralle 3 13 23? R3O7/285 ity of fire detectors connected across the conductors, 8 317/33means at the end of said conductors placing a constant load on thevoltage source and means adjoining the voltage source for detecting lossof said load resulting [56] References cued from a break in one of saidconductors.

UNITED STATES PATENTS 3,500,394 3/1970 Egesdal 340/409 1 Claim, 4Drawing Figures 1 1 Y VOLTAGE T 6 /7 "DETECTOR x 0 2 l 1 1 'f 6 1 7 1 B2 2 5 1 i VOLTAGE 5:; 9 9 DETECTOR 7 7 K P /0 8 2 4'! 3/ 3. v 7 DISCON-4 2 NECT/ON MONITOR FIRE DETECTING SYSTEM This invention relates to afire detecting system and more particularly to a fire detecting systemprovided with improved means for disconnecting detectors.

A conventional fire detecting system comprises a single central unit anda plurality of fire detectors connected in parallel between a pair ofconductors extending from the central unit. Each detector includes anormally open switch which is closed by heat or smoke to short-circuitboth conductors, and the central unit includes a power supply forsupplying an operation voltage between both conductors and an alarmdevice for sensing increase of current flowing through the conductorsdue to closure of the normally open switch in any of the detectors toproduce an alarm.

When either of the conductors is disconnected or broken off at someposition by an accident in such fire detecting system, the detectorsdisposed remote from this disconnected point become ineffective and suchdisconnection of the conductor is difficult to locate unless specialtesting means are used. As a way oflocating disconnection of theconductor at the central unit, a method of providing a bridge circuit inthe central unit and measuring a change of resistance between the outputterminals and a method of providing a special signal source at the endsof the conductors and receiving the signal at the central unit have beenproposed. However, such methods have required very complicated andcostly circuit arrangements.

Therefore, an object of this invention is to propose a fire detectingsystem provided with simple and inexpensive disconnection detectingmeans whereby any disconnecting of the conductors is automaticallylocated and information is provided at the central unit.

The above object can be accomplished in accordance with this inventionby inserting a Zener diode between the ends of both conductors tomaintain the voltage between the conductors at the Zener voltage andproviding means for detecting an increase of said voltage and producingan alarm at the central unit.

Other features and operation of the system of this invention will bedescribed in more detail hereinunder with reference to the accompanyingdrawings.

In the drawings:

' FIG. 1 is a block diagram representing an embodiment of the system ofthis invention,

FIG. 2 is a circuit diagram representing an example of the voltagedetector shown in block form in FIG.

FIG. 3 is a circuit diagram representing an example of the disconnectionmonitor 13 shown in block form in FIG. 1, and

FIG. 4 is a block diagram representing another embodiment of a system ofthis invention.

Throughout the drawings, like reference numerals are used to denote likecomponents.

Referring first to FIG. 1, the system of this invention includes acentral unit 1 and a plurality of fire detectors 2. Each of thedetectors 2 is symbolized as a normally open switch since it hasgenerally a switching element which is closed in response to detectionof heat or smoke to short-circuit a pair of conductors 5 and 6 whichextend from a pair of terminals 3 and 4 of the central unit 1. Betweenthe other ends of the conductors 3 and 4, inserted is a Zener diode 7according to this invention.

The central unit 1 includes a power supply 8 and a resistor 9 connectedin series with the terminal 3, a voltage detector 10 which senses avoltage rise across the resistor 9 to energize a relay element includedtherein, and a normally open switch which is to be actuated by the relayelement of the voltage detector 10 and an alarm generator 12 such asbell or buzzer connected in series with the relay contacts across thepower supply 8. The central unit 1 further includes a disconnectionmonitor 13 in accordance with the invention, and which is connected inseries with a diode 14 between the conductors 5 and 6.

An example of circuit configuration of the voltage detector 10 in FIG. 1is shown in FIG. 2. An electromagnetic winding 15 of the relay elementwhich actuates the normally open switch 11 is connected in series with asilicon controlled rectifier (hereinafter referred to as SCR) 16 betweenone end of the resistor 9 and the conductor 6, and a series connectionof a resistor 17 and a Zener diode 18 is connected in parallel to theseries connection of the winding 15 and the SCR 16. The connection pointof the resistor 17 and the zener diode 18 is connected to the emitterelectrode of a transistor 19 having a collector electrode connected tothe control electrode of the SCR 16 and also connected through aresistor 20 to the conductor 6. The base electrode of the transistor 19is directly connected to the other end of the resistor 9. Thus, a bridgecircuit is formed which includes the resistor 17, the Zener diode 18,the resistor 9 and the Zener diode 7. The Zener voltage of the Zenerdiode 18 is previously selected to be lower than that of the Zener diode7.

In operation, a voltage which may be somewhat higher than the Zenervoltage of the Zener diode 7 is applied from the power supply 8 throughthe conductors 5 and 6 to all detectors 2, but the voltage appliedpractically to the detectors 2 is limited to and maintained at the Zenervoltage of the Zener diode 7. Therefore, the Zener voltage of the Zenerdiode 7 is previously selected to be optimum for the detectors 2 toexhibit maximum sensitivity. In such a normal moni toring conditionwherein none of the detectors 2 is actuated, the base voltage of thetransistor 19 is substantially zero with respect to the emitter voltageand the transistor 19 is in a cut-off condition. Consequently, the SCR16 which is controlled by the collector voltage of the transistor 19 isalso in a cut-off conditon and no current flows through theelectromagnetic winding 15.

When one of the detectors 2 is actuated by a fire, the conductors 5 and6 are substantially short-circuited through a low impedance and,accordingly, the voltage across the resistor 9 is suddenly raised toabout the source voltage. Thus, the transistor 19 is driven intoconduction and the SCR 16 is also driven into conduction. Thus, theelectromagnetic winding 15 is energized to close the switch 11. Thisresults in actuation of the alarm 12.

An example of circuit configuration of the disconnection monitor 13 inFIG. 1 is shown in FIG. 3. A Zener diode 21 and a resistor 22 areconnected in series between the diode 14 and the conductor 6. A SCR 23has a control electrode connected to the connection point of the Zenerdiode 21 and the resistor 22. The SCR 23 is connected in series in aclosed circuit with a known alarm circuit 24 and a power supply 25, andthe cathode electrode of the SCR 23 is connected to the conductor 6. TheZener voltage of the Zener diode 21 is previously selected to be higherthan that of the Zener diode 7 but lower than the source voltage.Therefore, in the normal monitoring condition, the Zener diode 7 is inan ON-condition but the Zener diode 21 is in an OFF-condition, and nocurrent flows in the disconnection monitor 13.

When either the conductor or 6 is broken or cut, however, the Zenerdiode 7 is removed from the circuit and, therefore, the voltage betweenthe conductors 5 and 6 is suddenly raised to about the source voltage.Consequently, the Zener diode 21 conducts to raise the anode voltagethereof, and the SCR 23 is thus driven into conduction. Thus, the alarmcircuit 24 is actuated to give an alarm.

Referring then to FIG. 4, this embodiment of fire detecting systemincludes a single central unit 1 and a plurality of branches (twobranches in the drawing) of the fire detectors 2. Such configuration isoften used in practice for dividing the fire detectors into pluralgroups to facilitate location of a fire. The central unit 1 includes asingle power supply 8 but plural pairs of output terminals 3-1, 4-1,3-2, 4-2, from which respective pairs of conductors 5 and 6 extend.Between each pair of conductors 5 and 6 there are a plurality of firedetectors 2 and a terminating Zener diode 7 as in the case of FIG. 1.The branches of the detectors are provided with their respective voltagedetectors 10-1, 10-2, but with a common disconnection monitor 13.Instead of the switch 11 of FIG. 2, a parallel connection of switches11-1, 11-2, which are respectively actuated by the voltage detectors10-1, 10-2, are provided, so that the alarm 12 is actuated if any firedetector in any branch is actuated. Indication lamps may be providedrespectively in conjunction with the switches 11-1, 11-2, or voltagedetectors 10-1, 10-2, for indicating visually that branch which includesan actuated detector. Since the disconnection monitor 13 is connected incommon with all branches through respective diodes 14-1, 14-2, the

monitor 13 can indicate a discorinection occurring in any branch butcannot determine the particular branch in which the disconnection hasoccurred. For this purpose, a disconnection monitor 13 may be providedin each of the branches or indicating lamps may be arranged so as to beexcited in response to conduction of the diodes 14-1, 14-2,

As described in the above, .disconnection of distributed conductors offire detecting system can be automatically determined at the centralunit by simple and inexpensive additions, such as a Zener diode 7 and adisconnection monitor 13. Moreover, it is unnecessary to sufficientlystabilize the source voltage because of the inclusion of the Zener diode7. This is especially advantageous when the fire detectors are those ofionization smoke detector types which require a wellstabilized dc.voltage source.

It should be noted that the above mentioned embodiments are onlyexamples presented as an aid for explanation of this invention andvarious variations, modifications and changes can be made withoutdeparting from the scope and spirit of this invention. For example,various circuit configurations can be considered by those skilled in theart for the voltage detector 10 and disconnection monitor 13, other thanthe circuits in FIGS. 2 and 3.

What is claimed is:

l. A fire detecting system comprising a voltage source, a pair ofconductors extending from both terminals of said voltage source, aplurality of fire detectors connected in parallel between said pair ofconductors, each detector being provided with switch means forshort-circuiting said pair of conductors upon actuation, a first alarmand means for sensing an increase in the current flowing through saidconductors and actuating said first alarm, a first zener diode connectedbetween the other ends of said pair of conductors, a disconnectionmonitor including a second zener diode connected between said pair ofconductors, a second alarm, means interconnected with said second zenerdiode for sensing an increase in the current flowing through said secondzener diode, and a connection between the last said means and saidsecond alarm to actuate said second alarm, the zener voltage of saidsecond zener diode being selected to be higher than the zener voltage ofsaid first zener diode but lower than the terminal voltage of saidvoltage source.

1. A fire detecting system comprising a voltage source, a pair ofconductors extending from both terminals of said voltage source, aplurality of fire detectors connected in parallel between said pair ofconductors, each detector being provided with switch means forshort-circuiting said pair of conductors upon actuation, a first alarmand means for sensing an increase in the current flowing through saidconductors and actuating said first alarm, a first zener diode connectedbetween the other ends of said pair of conductors, a disconnectionmonitor including a second zener diode connected between said pair ofconductors, a second alarm, means interconnected with said second zenerdiode for sensing an increAse in the current flowing through said secondzener diode, and a connection between the last said means and saidsecond alarm to actuate said second alarm, the zener voltage of saidsecond zener diode being selected to be higher than the zener voltage ofsaid first zener diode but lower than the terminal voltage of saidvoltage source.